Influence of external shear and strain on baroclinic vortex alignment
Seminar Room 1, Newton Institute
The influence of an external strain (or shear) field on the evolution of two identical vortices at different depths is investigated in a two-layer quasi-geostrophic model. The external deformation can include a time-varying component. Using point vortex modeling, symmetric equilibrium positions for the vortex doublet are identified and their stability is computed. A time- varying external field can induce resonances on both harmonic and subharmonic frequencies. A slow time amplitude equation is then computed for the vortex trajectories around their equilibrium positions ; its solutions are compared successfully to numerical simulations of a baroclinic point vortex model. The evolution from regular to chaotic trajectories when the external field amplitude grows, is analyzed. Finite-area vortices exhibit the classical regimes of alignment or of co-rotation when submitted to external strain and rotation, but they can also remain stationary at the location of the neutral points for the equivalent point vortices, or oscillate around these positions. Another regime is evidenced, in which originally very distant vortices are advected towards the center of the plane by the large-scale flow, and finally merge. Thus, in a dense field of coherent structures, neighboring vortices can favor the vertical alignment of two central vortices, despite the erosion that they induce on these vortices.